In recent years, in association with the progress in the fields of image processing technology or various types of device technology as well as with automation in commerce, industry, and other industrial fields, an image identifying technology for identifying whether a document image is a particular image or not has been widely utilized.
Examples of a conventional type of image identifying apparatus include, for instance, ATM (Automatic teller Machine) used in banks or other financial institutions, a bar code reader used in a process of checking printed picture in a production line of a printing factory, an apparatus for assorting merchandise coupons, and a mark reader used in various types of assortment work.
In this type of image identifying apparatus as described above, discrimination (identification) as to whether an document image is a particular image or not is executed by irradiating light to a document image, receiving the reflected light with an image sensor in which a CCD element is used, generating an analog signal by executing photoelectric conversion, amplifying the analog signal to a specified range with an amplifier, then converting the analog signal to a 2-valued digital signal using a threshold value preset by an A/D convertor, reading image data of the document image, and then comparing the image data to a preset pattern (digital signal) for discrimination. In other words, density data (image data) is generated by optically reading a document image and discrimination of the document image is mad based on this density data.
However, with the image identifying apparatus using a CCD element as a conventional type of image sensor therein, it is possible to carry out a high performance image identification by reading an image at a high precision with a CCD element, but as the CCD element is expensive, price of the apparatus also becomes expensive, and the technology as described above is not suited to realization of a cheap system.
Also some users sometimes hope to carry out image identification at a certain degree of accuracy with a cheap system which is not so high in its performance, but any image sensor that can satisfy the demands as described above in a scope of practical applications is not available. Especially in relation to the needs for image identification in a state where types of document image are limited, development of an image sensor which enables image identification in low cost at a high speed has been desired.
On the other hand, a reflection type of sensor with an amplifier is available as an image sensor, and this type of sensor is cheaper as compared to that using a CCD elements therein, but as an input analog signal is converted to a digital signal based on whether said input analog signal is larger than a threshold value (threshold level) or not, it is necessary to execute at a high precision gain adjustment for amplifying an analog signal to a specified range in order to precisely read a document image, and the gain adjustment is very complicated, which is disadvantageously in practical operation. For this reason, this type of technology is suited for checking simply whether an identical object or an identical marking exists or not, but it is not suited to a sensor to discriminate a detailed pattern with a certain degree of accuracy.
Also in the conventional type of reflection type sensor with an amplifier as described above, reflected light is focused via a lens to a photoelectric conversion element such as a phototransistor and also to provide a certain space corresponding to the focus range between the lens and the photoelectric conversion element, which makes it difficult to make a sensor compact.
Also when, for instance, a quantity of light from a light source to irradiate light onto a document image changes due to elapse of time, an analog signal outputted from the photoelectric conversion element becomes generally smaller, so that a circuit to maintain a quantity of light from the light source is required to read an document image at a high precision and for this reason it is difficult to make a sensor.